34 SCIENTIFIC AGRICULTURE. 



certain gases, &c. : this is a feeble light, and is only visible in 

 darkness. 



CALORIC. 



Caloric is the substance or agent which is thrown off by 

 heated or burning bodies, and which produces the sensation 

 of heat : in common language it is the word used to express 

 both the cause and the effect. This agent possesses no appre- 

 ciable weight. Although it must be substance^ or material 

 in its nature, still a body when highly heated or charged 

 with caloric, is not sensibly heavier raan when cold. 



Caloric appears to exist in all bodies. Heat and cold are 

 only relative terms ; when a body is so cold as to produce the 

 sensation of coldness to our touch, we call it cold; on the 

 contrary, when it produces the sensation of warmth, we call it 

 warm, although the absolute temperature may be the same 

 in both cases. Caloric always tends to seek an equilibrium: 

 that is, it constantly passes from the hotter to the colder 

 bodies: if a piece of ice at 32 be carried into an atmosphere 

 where the temperature is 60 below 32, it will, in changing 

 its temperature to that of the surrounding air, give off 60 of 

 heat : this illustration is sufficient to prove that the ice really 

 contains heat. 



The expansive power of heat is another property which 

 involves many important facts : when caloric enters a body, it 

 is supposed that a mutual repulsion of its particles takes place, 

 so as to partially overcome their cohesive power, and render 

 the body less dense. All bodies expand by heat, the degree 

 of this expansion, however, differs widely in different bodies. 

 The expansibility of fluids is greater than that of solids, with 

 equal degrees of heat. 



All gases expand nearly equally with the same degrees of 

 heat : this is not the case, however, with the solids and liquids. 

 Some bodies are much better conductors of caloric than others : 



